docs: def mono_resolution(
      grooves_per_mm, slit_width, focal_length, output_color, output_units="wn"
  ) -> float:
      """Calculate the resolution of a monochromator.
  
      Parameters
      ----------
      grooves_per_mm : number
          Grooves per millimeter.
      slit_width : number
          Slit width in microns.
      focal_length : number
          Focal length in mm.
      output_color : number
          Output color in nm.
      output_units : string (optional)
          Output units. Default is wn.
  
      Returns
      -------
      float
          Resolution.
      """
      d_lambda = 1e6 * slit_width / (grooves_per_mm * focal_length)  # nm
      upper = output_color + d_lambda / 2  # nm
      lower = output_color - d_lambda / 2  # nm
      return abs(
          wt_units.converter(upper, "nm", output_units)
          - wt_units.converter(lower, "nm", output_units)
      )